Several new manufacturing processes, commonly referred to as solid free form fabrication (SFF), have recently emerged to build parts point-by-point and layer-by-layer. These processes were originally developed for use as a shortcut to making models, molds and dies, and prototype parts. These technologies entail the production of freeform solid objects directly from a computer-created model without part-specific tooling or human intervention. Solid free form fabrication also has potential as a cost-effective production process if the number of parts needed at a given time is relatively small. Use of SFF could reduce tool-making time and cost, and provide the opportunity to modify tool design without incurring high costs and lengthy time delays. A SFF process can be used to fabricate certain parts with a complex geometry which otherwise could not be practically made by traditional fabrication approaches such as machining. The reference book written by P. F. Jacobs (entitled "Rapid Prototyping and Manufacturing: Fundamentals of Stereo Lithography," McGraw Hill Pub. New York, 1992) provides a good overview on various SFF methods.
The SFF technology has three levels of sophistication: The first is the ability to generate models or prototypes that clearly show the part design concept in three dimensions. All SFF techniques developed so far (e.g., stereo lithography, selective laser sintering, solid ground curing, 3-D printing, laminated object manufacturing, fused deposition modeling, and recursive masking and depositing, etc.) are capable of creating such models. The second level is the ability to produce parts that have acceptable dimensions and tolerances, and sufficient strength for preliminary evaluation in a simulated service environment. Although some progress has been made in attempting to achieve this ability, the parts produced with no fiber reinforcement or with only short fibers, remain to be of inadequate structural integrity.
The third level is the ability to produce parts having high structural integrity and good dimensional tolerances so that they can be placed in real operating systems. Little has been done toward fabricating SFF parts with high structural integrity. Some preliminary attempts have been made to use stereo lithography based techniques to fabricate both short and continuous fiber reinforced, UV-curable resins. In most cases, only composites with excessively low volume fractions of fibers were obtained and, hence, the resulting composites have exhibited low strength and stiffness. Furthermore, in such stereo lithography based techniques, only a laser- or UV-curable resin can be employed as the matrix material for a composite.
Fiber reinforced composites are known to have great stiffness, strength, damage tolerance, fatigue resistance, and corrosion resistance. However, currently available SFF technologies, in their present forms, do not lend themselves to the production of continuous fiber composite parts. We have discovered that selected SFF approaches (such as fused deposition modeling) could be modified and integrated with textile structure-forming operations (such as selected steps in braiding, weaving, stitching, and knitting) to produce parts on an essentially layer-by-layer basis. The parts produced by such a combination of SFF and textile operations, being of continuous fiber reinforced composite, are of superior structural integrity. The new processes represent a major step forward toward achieving the highest level of sophistication in SFF. They would likely become competitive, preferred, or possibly the only approaches for fabricating certain composite parts of a complex geometry.
Fused Deposition Modeling (FDM) and Shape Deposition Manufacturing (SDM) are two relatively flexible and versatile SFF approaches. In the conventional FDM (e.g. S. S. Crump, U.S. Pat. No. 5,121,329 (1992)), a continuous filament of a solid thermoplastic polymer, wax, or metal is heated to become liquid and extruded through a nozzle, which deposits the liquid on a base member. The relative motions along the "X"-axis and "Y"-axis between the nozzle and the base member is computer-controlled to trace out the cross sections of a part on a layer-by-layer basis. The material is heated to only slightly above the melting point, so that solidification occurs within seconds after exiting the nozzle.
In the SDM technology that is under active development, material deposition and material removal processes are integrated to build an object. Layer segments are deposited as near-net shapes and then machined to net-shape before additional material is deposited. This method was included in the discussion of an article written by L. E. Weiss ("Solid Freeform Fabrication Processes," in the Proc. of NSF Workshop on Design Methodologies for Solid Freeform Fabrication," Jun. 5-6, 1995, Pittsburgh, Pa.). Both FDM and SDM typically involve building a prototype part with a neat material, such as a resin or metal, without any fiber reinforcement. No prior art has been reported on building continuous fiber reinforced parts using a FDM- or SDM-based technology.
Numerous prior art methods are being practiced to produce continuous fiber reinforced composites. All of these methods have disadvantages or shortcomings. For example, the hand lay-up process is labor-intensive and the quality of the resulting composite part depends highly upon the skills of an operator. The combined process of prepreg preparation, cutting, lay-up, vacuum bagging, and autoclave or press curing is notoriously tedious, lengthy, and energy-intensive. In resin transfer molding (RTM), a dry reinforcement material, originally in the forms of roving, mat, fabric, or a combination, is cut and shaped into a preform. The preform is then pre-rigidized by using a small amount of fast-curing resin to hold its shape during the subsequent operations. The preform is then placed in a mold, the mold is closed and resin is then injected into it. Resin must flow through the small channels inside a normally tightly-configured preform, expelling the air in the mold cavity, impregnating the preform and wetting out the fibers. The RTM process suffers from several drawbacks. First, it requires execution of two separate processes: preform preparation and resin impregnation. Complete impregnation of a dense or large-sized preform by a viscous resin can be very difficult. Second, it requires utilization of a mold, which is normally quite costly. Third, RTM is not suitable for fabricating complex-shaped parts (e.g. part with a hollow cavity).
Processes such as filament winding and pultrusion can be highly automated. However, filament winding is essentially limited to fabrication of convex-shaped hollow structures such as pressure vessels. Pultrusion can produce a variety of reinforced solid, tubular, or structural profiles. Unfortunately, these structures are essentially limited to be of a constant cross-section. Both filament winding and protrusion are not well-suited to production of complex-shaped parts. An overview of various composite processing techniques is given in a reference book written by one of us (B. Z. Jang, "Advanced Polymer Composites: Principles and Applications," ASM International, Materials Park, Ohio, December 1994).
In summary, currently available SFF technologies, generally speaking, do not lend themselves to the production of continuous fiber composite parts. In general, current composite processing techniques are not capable of producing parts of a complex geometry, or producing parts of a specified geometry directly from a computer-aided design. Accordingly, it is desirable to develop a process and apparatus that can be used to fabricate continuous fiber reinforced composite parts of high structural integrity and complex geometry. It is further desirable that the process also has the capability of producing a three-dimensional object automatically in response to the computer-aided design of the object.